Electrically conductive materials formed by electrophoresis

ABSTRACT

A method of forming an electrically conductive composite is disclosed that includes the steps of providing a first dielectric material and a second conductive material that is substantially dispersed within the first dielectric material; and applying an electric field through at least a portion of the combined first dielectric material and second conductive material such that the second conductive material undergoes electrophoresis and forms at least one electrically conductive path through the electrically conductive composite along the direction of the applied electric field.

PRIORITY INFORMATION

This invention claims priority to U.S. patent application No. 13/272,527filed in the United States Patent and Trademark Office on Oct. 13, 2011.

BACKGROUND

The invention generally relates to conductive polymeric and elastomericmaterials for use in a wide variety of applications, including withoutlimitation, conductive adhesives, conductive gaskets and conductivefilms.

The design of an electrically conductive pressure sensitive adhesive(PSA), for example, for has long presented challenges at least becauseadhesive strength and flexibility generally decrease with increasedelectrical conductivity. The materials that are typically used (added)to provide good electrical conductivity are generally less flexible andinhibit adhesion. A conventional way to prepare a conductive coating isto fill a polymeric material with conductive particles, e.g., graphite,silver, copper, etc., then coat, dry and cure the polymeric binder. Inthese cases the conductive particles are in such a concentration thatthere is a conductive network formed when the particles are each inphysical contact with at least one other neighboring particle. In thisway, a conductive path is provided through the composite.

For pressure sensitive adhesives, however, if the particle concentrationis high enough to form a network in which particle-to-particle contactis maintained then there is little chance that the polymer (e.g.,elastomer) system of the PSA component is present in high enoughconcentrations to flow out to make surface-to-surface contact betweenthe substrates and an electrode, i.e., act as an adhesive. Conversely,if the PSA component is sufficient concentration to make sufficientsurface contact to the substrate, it would have to interrupt adjacentconductive particles such that particle-to-particle contact isdisrupted.

Another type of electrically conductive PSA includes conductivespherical particles with diameters equal to or greater than thethickness of the PSA. In this fashion the signal or current may becarried along the surface of the particles, thus providing current flowanisotropically in the z dimension of the adhesive. The continuity ofthe adhesive however, may be compromised.

Salts, such as sodium or potassium chloride, readily dissolve when in anaqueous medium, and their ions dissociate (separate into positive andnegative ions). The dissociated ions may then convey an electricalcurrent or signal. For this reason, salts have long been added to water,which then may be added to polymeric and elastomeric materials, toprovide good electrical conductivity. For example, U.S. Pat. No.6,121,508 discloses a pressure sensitive adhesive hydrogel for use in abiomedical electrode. The gel material is disclosed to include at leastwater, potassium chloride and polyethylene glycol, and is disclosed tobe electrically conductive. U.S. Pat. No. 5,800,685 also discloses anelectrically conductive adhesive hydrogel that includes water, salt, aninitiator or catalyst and a cross linking agent. The use of suchhydrogels however, also generally requires the use of a conductivesurface at one side of the hydrogel (away from the patient) that iscapable of receiving the ionically conductive charge, such assilver/silver chloride, which is relatively expensive.

While these hydrogel/adhesives can have good electrically conductiveproperties, they often have only fair adhesion properties. Anotherdownside is that the electrical conductivity changes with changing watercontent, such as changes caused by evaporation, requiring that thehydrogels be maintained in a sealed environment prior to use, and thenused for a limited period of time only due to evaporation.

U.S. Pat. No. 7,651,638 discloses a water insensitive alternatingcurrent responsive composite that includes a polymeric material and apolar material (such as an organo salt) that is substantially dispersedwithin the polymeric material. The polymeric material and the polarmaterial are chosen such that they each exhibit a mutual attraction thatis substantially the same as the attraction to itself. Because of this,the polar material neither clumps together nor blooms to a surface ofthe polymeric material, but remains suspended within the polymericmaterial. This is in contrast to the use of salts in other applicationsthat is intended to bloom to the surface (to provide a conductive layeralong a surface, e.g., for static discharge)

The composites of U.S. Pat. No. 7,651,638, however, remain dielectricsand have high resistance, and are therefore not suitable for use incertain applications, such as providing electrical stimulus to a humansubject (such as is required during defibrillation and/or TranscutaneousElectrical Nerve Stimulation, etc.) due to the high resistance of thematerial. While such composites may be used for detecting smallbiological electric signals from a patient, a problem therefore, canarise when a patient undergoes a defibrillation procedure because thehigh resistance could prevent the charge overload from dissipating in atimely enough fashion as per AAMI EC12-2000-4.2.2.4, which is directedto defibrillation overload recovery (DOR). This failure to dissipate thecharge may lead to uncertainty as to whether or not the defibrillationprocedure has corrected the distress and therefore whether any furthercharge needs to be given to the patient.

U.S. Pat. No. 5,082,595 discloses an electrically conductive pressuresensitive adhesive that includes carbon particles, and the conductiveadhesive is disclosed to be prepared by incorporating black filler(carbon) into the pressure sensitive adhesive in such a manner as toimpart electrical conductivity, yet have a concentration low enough toavoid adversely affecting the physical properties (such as tack) of theadhesive. In particular, this patent states that a slurry of the carbonblack in an organic solvent is formed under mild agitation or stirringin the absence of high shear to preserve the structures carbon black mayform and to improve wetting of the carbon black. Such a composite,however, may not provide sufficient adhesiveness and conductivity incertain applications. Nor may such structures be discreetly placed toform conduction sites only at specific locations within a continuousadhesive.

There remains a need therefore, for a composite for use as a conductivepolymeric material that provides electrical conductivity withoutcompromising the desired properties of the polymeric material.

SUMMARY

In accordance with an embodiment, the invention provides a method offorming an electrically conductive composite comprising the steps ofproviding a first dielectric material and a second conductive materialthat is substantially dispersed within the first dielectric material;and applying an electric field through at least a portion of thecombined first dielectric material and second conductive material suchthat the second conductive material undergoes electrophoresis and formsat least one electrically conductive path through the composite alongthe direction of the applied electric field.

In accordance with another embodiment, the invention provides anelectrically conductive material comprising a dielectric material andconductive particles within the dielectric material, wherein theconductive particles are aligned to form conductive paths through thecomposite by electrophoresis.

In accordance with further embodiments, the dielectric material may be apressure sensitive adhesive, the conductive particles may be formed ofany of carbon powder, flakes, granules or nanotubes, and the conductiveparticles may have densities within the range of about 0.35 g/cm³ andabout 1.20 g/cm³.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description may be further understood with reference tothe accompanying drawings in which:

FIG. 1 shows an illustrative diagrammatic view of a composite inaccordance with an embodiment of the invention prior to electrophoresis;

FIG. 2 shows an illustrative diagrammatic view of the composite of FIG.1 in the presence of an electric field sufficient to causeelectrophoresis in the composite;

FIG. 3 shows an illustrative diagrammatic view of the composite of FIG.2 after the electric field has been applied and removed;

FIG. 4 shows an illustrative diagrammatic view of the composite of FIG.3 used for conducting electricity from one side of the composite to theother side of the composite;

FIGS. 5A-5C show illustrative diagrammatic views of the composite ofFIG. 1 at successive moments after a direct current (DC) overchargeelectric field is applied showing the electrophoresis activity;

FIGS. 6A and 6B show illustrative diagrammatic views of the composite ofFIG. 1 at successive moments after an alternating current (AC)overcharge electric field is applied showing the electrophoresisactivity;

FIGS. 7 and 8 show illustrative diagrammatic views of the conductivepathways formed in the polymeric material;

FIGS. 9 and 10 show illustrative micro-photographic views of compositesof the invention at different magnifications; and

FIGS. 11 and 12 show illustrative diagrammatic views of composites of afurther embodiment of the invention before and after electrophoresisproviding electrical conductivity in multiple dimensions.

The drawings are shown for illustrative purposes only and are not toscale.

DETAILED DESCRIPTION

Applicants have discovered that conductive materials may be formed byelectrophoresis whereby conductive particles (e.g., 5% by weight carbonparticles) within a dielectric material (e.g., a pressure sensitiveadhesive) migrate when subjected to an electric field by aligning withthe field to form conductive pathways through the composite.

The requirements for the dielectric material (e.g., polymeric material)and the conductive material include that the materials interact in sucha way that the conductive material does not bloom to a surface of thebinder material. If conductive material has a surface energy greaterthan that of the dielectric material, then the conductive materialshould remain suspended within the dielectric material yet not be insufficient concentrations to provide particle-to-particle electricalconductivity through the material prior to the application of anelectric field.

FIG. 1 for example, shows a composite 10 in accordance with anembodiment of the invention that includes a dielectric material 12 andconductive particles 14 dispersed within the dielectric material 12.This may be achieved, for example, by introducing the conductivematerial (while dispersed in an evaporative continuous liquid phase)into the liquid polymeric material and then permitting the liquid phaseof the dispersion of conductive particles to evaporate leaving theconductive material within the polymeric material. In accordance with anembodiment of the invention, the polymeric material may, for example, bean acrylic adhesive such as may be represented as

Where R may vary and may be any of an ethyl, or a butyl or a2-ethylhexyl or other organic moiety, and n is a number of repeatingunits. For example, the polymeric material may be a FLEXcon V95 pressuresensitive adhesive as sold by FLEXcon Company, Inc. of Spencer, Mass.

As shown in FIG. 2, when an electric field 18 (e.g., 5, 10, 50, 100, 200volts or higher AC or DC) is applied to the composite at conductors 20,22, the conductive particles 14 undergo electrophoresis and will alignwith the electric field, forming a conductive path through thecomposite. As shown in FIG. 3, when the field is removed, the conductiveparticles 14 remain in place forming the conductive path. The compositemay then be used to conduct electricity between, for example conductors26 and 28 as shown in FIG. 4.

The conductive particles should have a surface energy that is at leastslightly greater than that of the dielectric material to ensure that thedielectric material sufficiently wets the surface of the conductiveparticles. The density and surface area of the conductivity of theparticles 14 are important considerations. Applicants have found, forexample, that carbon (e.g., graphite powder, flakes, granules, nanotubesetc.) having densities in the range of, for example, about 0.35 g/cm³ toabout 1.20 g/cm³, and preferably between about 0.5 g/cm³ to 1.0 g/cm³,are suitable for use as the conductive material. The surface energy ofthe graphite is, again, preferably higher than that of the dielectric toensure sufficient wetting of the surfaces of the particles 14. In theabove example, the graphite particles have a specific surface energy of55 dynes/cm and the dielectric disclosed above has a surface energy of alittle less than 40 dynes/cm.

FIGS. 5A-5C show the process of the electrophoresis that occurs uponovercharging in more detail. As shown in FIG. 5A, when a DC voltagepotential is applied, e.g., 5, 10, 50, 100 or 200 volts or higher, aparticle 14 a that is near the surface aligns in the z-direction. Oncethis occurs, the inner end 16 a of the particle 14 a is now closer tothe opposing surface (as also shown in FIG. 5A), causing the charge onthe inner end 16 a to be slightly higher than the charge on thesurrounding inner surface of the composite. This causes another nearbyparticle 14 b to be attracted to the inner end 16 a of the particle 14 aas shown in FIG. 5B. The inner end of the particle 14 b is now highlycharged, causing nearby particle 14 c to be attracted to it as shown inFIG. 5C. Further particles (e.g., 14 d as shown) are further attractedto the ends of the thus formed path. This all occurs rapidly and theattractive/aligning force causing the electrophoresis is believed tobecome stronger as the path is formed.

As shown in FIG. 6A, when an AC voltage is applied (again, e.g., 5, 10,50, 100 or 200 volts or higher), the particles 15 a and 15 b form alonga first side of the composite 12 that has a positive voltage applied toit at a first conductor 31. When a positive voltage charge is thenapplied at the opposite conductor 33, the conductive particles 15 c and15 d then begin to agglomerate from the lower side of the composite asshown in FIG. 6B. By thus alternating the agglomeration process betweenopposite sides, the AC overvoltage causes a path to be formed thatessentially meets in the middle.

Regardless of whether the charge is DC or AC, the higher the voltage,the faster the particles align, and with a relatively low voltage (e.g.,about 5 volts or higher), the particles align more slowly, but do stilleventually align. This agglomeration phenomenon may be referred to aselectrophoretic (in the presence of a DC field) or dielectrophoretic (inthe presence of an AC field), both of which are referred to herein as anelectrophoresis process.

As shown in FIG. 7, following application of a voltage as discussedabove over a small area of the composite, multiple conductive paths 38will be formed through the composite, wherein each conductive path isformed by aligned conductive particles. As shown in FIG. 8, groups ofsuch conductive paths 40, 42, 44 may be separated from one anotherthrough selective application of distinct electric fields, permittingselected areas of the composite to be electrically conductive, whileother areas 46 of the composite exhibit a high dielectric constant andare therefore not electrically conductive.

In accordance with an embodiment, in one example, to a liquid sample ofFLEXcon's V-95 acrylic PSA was added 5% by weight (solids of the V-95FLEXcon and Arquad blend) of a carbon particle (the Aquablack 5909carbon particles from Solution Dispersions Inc., Cynthiana Ky.), whichwas uniformly dispersed within the polymer. This mixture was coated ontoa 2 mil (50 micron) siliconized one side PET film, dried and cured for10 min in a 160° F. vented laboratory oven, to a dried deposition of 2mil (50 micron). Upon placing the carbon particle in the V-95 acrylicadhesive composite between two electrodes, and electrically charging theelectrodes, conductive structures were formed. It has further been foundthat the composite has a Z dimension directionality to the signalreceptivity. This maintenance of Z dimensionality allows this adhesiveto be used in applications as disclosed in U.S. Patent ApplicationPublication No. 2010-0036230 (the disclosure of which is herebyincorporated by reference in its entirety), which teaches the formationof a bio-sensor array fashioned with one continuous layer of adhesive,the disclosure of which is hereby incorporated by reference in itsentirety.

Composites in accordance with certain embodiments of the presentinvention, begin with substantially separated particles uniformlydispersed within, for example, an adhesive. In a subsequent step, anelectric field is applied to form the conductive structures. This is adecided advantage as it allows the placement of conductive structures inthe Z dimension at specific X,Y, locations thus allowing for a specificpoint to point electrical contact.

Again, with reference to FIG. 7, multiple parallel paths 48 may beformed simultaneously upon application of a wide electric field. Thedistance between the paths will depend on the thickness of the material12 and the concentration of conductive particles as well as any surfaceirregularities on the outer surfaces of the material 12. As shown inFIG. 8, discrete sets of such paths 40, 42, 44 may be separated from oneanother providing areas of non-conductive portions 46 there-betweenthrough application of electric fields in discrete areas (adjacent setsof paths 40, 42, 44).

The following Examples demonstrate the effect of the conductive particleaddition to the binder material discussed above.

EXAMPLE 1

To a liquid sample of FLEXcon's V-95 acrylic PSA, is added a polarmaterial, Arquad HTL-8 (AkzoNobel), 20% by weight on solids, to this 5%by weight (solids of the V-95 and Arquad blend) of a carbon particle(Aquablack 5909 from Solution Dispersions Inc., Cynthiana Ky.), whichwas uniformly dispersed and was designated as Sample 1. This mixture wascoated on a 2 mil (50 micron) siliconized one side PET film, dried andcured for 10 min in a 160° F. vented laboratory oven, to a drieddeposition of 2 mil (50 micron).

Also prepared at this time was the composite of just the V-95 acrylicadhesive and the Arquad (20% by solids weight), no carbon, as per thedisclosure in U.S. Pat. No. 7,651,638 (the disclosure of which is herebyincorporated by reference in its entirety), and was designated as Sample2.

This mixture was also 2 mil (50 microns) siliconized on one side of aPET film, dried and cured for 10 min in a 160° F. vented laboratoryoven, to a dried deposition of 2 mil (50 microns) and was designated asSample 2.

Similarly a third sample was prepared consisting of only V-95 acrylicadhesive and 5% carbon, no polar material (Arquad), processed in thesame manner as for samples 1 and 2, and was designated as Sample 3.

All three samples were tested on a conductive base material consistingof a carbon filled polymeric film with a surface resistance of ˜100ohms/square, using the experimental product designated EXV-215, 90PFW(as sold by FLEXcon Company, Inc. of Spencer, Mass.). The samples weretested using a QuadTech LCR Model 1900 testing device sold by QuadTech,Inc. of Marlborough, Mass.

In particular, all three samples were tested as per AAMIEC12-2000-4.2.2.1 (modified) and AAMI EC12-2000-4.2.2.4. The AAMIEC12-2000-4.2.2.1 test has an upper limit of 3000 Ohms for the face toface double adhesive part of the test, for a single point and a maxaverage (12 test samples) of 2000 Ohms.

The AAMI EC12-2000-4.2.2.4 calls for retaining less than 100 mV in 5 secafter a 200 DC volt charge, again using a face to double adhesive layer.

Note the Table 1 below, which shows impedance (EC 12-2000-4.2.2.1)tested first; DOR (EC 12-2000-4.2.2.4) was run next on the same samples.

TABLE 1 EC12-2000- EC12-2000- Sample 4.2.2.1 (20 Hz) 4.2.2.4 Sample 160K Ohms (fail) 0.0 volts in less than 5 sec. (pass Sample 2 80K Ohms(fail) 150 volts after 5 sec. (fail) Sample 3 40M Ohms (fail) 0.0 voltsin less than 5 sec. (pass)

EXAMPLE 2

To determine the signal receptivity of this invention, the samplesprepared for Example 1 were tested in accordance to the procedureoutlined below. The samples used in testing as per AAMIEC12-2000-4.2.2.1 were used connected in series to a Wave Form Generator(Hewlett Packard 33120A 15 MHz Function/Arbitrary Waveform Generator)and in series an Oscilloscope (BK Precision 100 MHz Oscilloscope 2190).Samples were tested at 3, 10 and 100 Hz; results are given below inTable 2 in % of transmitted signal received.

TABLE 2 Sample 1 Sample 2 Sample 3  3 Hz 95+% 95% No signal 10 Hz 95+%95% No signal 100 Hz  95+% 95% No signal

EXAMPLE 3

Samples that passed the DOR test (AAMI EC12-2000-4.2.2.4) were retestedfor impedance as per AAMI EC12-2000-4.2.2.1 (modified), upon rechecking,samples 1 & 3 had a remarkable change. Samples 1 and 3 now had animpedance of less than 1 K Ohms. In sample 2, the signal receptivemedium was unchanged post DOR test; only those samples with thedispersed conductive particles changed. Further, the resulting lowerimpedance was still anisotropic, i.e., in the Z direction (notingExample 4 as to how the anisotropic property was determined). Inaddition the parallel capacitance (CP) of the post DOR material actuallyincreases as the Z impedance decreases, as shown below in Table 3.

TABLE 3 Ohms DC Resistance (Z direction) CP Farads Ohms Sample 1 60K11.0 nF   80K pre-DOR Sample 1 860 61.6 nF 790 post-DOR Sample 3 13M0.06 nF 100⁺M pre-DOR Sample 3 1.9K  41.2 nF 1.45K post-DOR

EXAMPLE 4

The anisotropic property was validated by the following test procedure.Signals at 3, 10, 100, Hz were generated, and fed to a first coppershunt, which was placed on the conductive adhesive. A second coppershunt was placed on the same conductive adhesive ˜0.004″ (100 microns)apart from the first shunt, which was connected (in series) to anoscilloscope. The base substrate was a dielectric material (PET film)

If the Sample 1 adhesive was isotropic it would have been expected topick up a signal on the oscilloscope. If the Sample 1 adhesive wasanisotropic it would have been expected that no signal would be receivedon the oscilloscope. The result was that no signal was detected.

The electrophoresis result does not appear to rely on the presence ofthe polar material in the composite. It is believed that the carbonparticles are agglomerated by the electric field applied during the DORtest; that the electric field generated by the 200 DC volts beingapplied across the conductive particle containing SRM and/or theconductive particles just with a PSA (no polar organo-salt), issufficient to cause the particles to agglomerate together, possibly byinducing an opposite charge on nearby particles.

The agglomerated structures spanning from one electrode to the other arethe reason an anisotropic conductive PSA is formed. To examine theseagglomerations, reference is made to FIG. 9, which show at 50 an in situformed conductive structure as per this invention. In particular, FIG. 9shows a 10× view looking down from the top of a conductive structure.The dark areas are the agglomerated particles the lighter arearepresents particle poor areas, i.e., places from which particlesmigrated.

This particle migration effect can be shown in more detail by looking atFIG. 10, which shows at 52 a 100× magnification of a conductivestructure, again looking down from the top, but focused more towards theedge, showing the lighter, particle poor, areas. The clear material isthe continuous medium, in this case a PSA, FLEXcon's V-95 acrylicadhesive. Note the striations or grooves in the clear V-95 acrylicadhesive, and also note that the few particles remaining are a linedwith the striations. The starting material was a uniform particledistribution in continuous medium, thus under the electric fieldgenerated by the DOR test, the particles move together to form theconductive structure. Again, this agglomeration phenomenon may bereferred to as electrophoretic or in the case of an AC electric field,dielectrophoretic effect, both of which are referred to herein as theelectrophoretic process.

It is significant; however, that in this case the agglomeration occursin a non-aqueous high viscosity medium. In accordance with the presentinvention, the continuous medium is a dielectric and is in full contactwith the conductive particles (at the particle loading levels) and themedium is a viscoelastic material, i.e., has a very high viscosity, fiveplus orders of magnitude higher (as measured in centipoises) than waterdispersions (often measured in the only the 10s of centipoises).

Again, what is postulated here is that, as in the case of particleagglomeration via an electric field in an aqueous continuous medium, aslight charge is induced on a nearby particle near an electrode. Withthe continuous medium being less polar and more dielectric than waterhowever, a greater charge build-up can occur on a particle in theelectric field.

With water as the continuous medium the higher polarity would mitigatethe charge build up, further if the applied electric field wereincreased (higher voltage) electrolysis of the water would become acompeting complication. With a PSA (e.g., FLEXcon's V-95 acrylicadhesive) as the continuous medium there is much less charge mitigationand no substantial electro-chemical process that occurs.

This charge build-up on the particle increases the attractive forcesbetween the particle and the electrode, thus drawing the particle to theelectrode in spite of the higher viscosity of the continuous medium.Further, the first particle that reaches the electrode forms anincremental high spot on said electrode thus the electric field is movedcloser to the other electrode, as more particles join the agglomerationthe field strength increase as the distance to the opposite electrodedecreases, thus accelerating the agglomeration growth.

The DOR test involves a plane to plane electrode arrangement; after afew conductive structures are formed therefore, the electric fieldbetween the two electrodes is mostly dissipated due to the contactsalready made between the electrodes. Thus the first structure will form,where there is one spot where the two planes are closer to one anotheror there is an uneven distribution of carbon such that a slightly higherdensity of the conductive particles are at one increment, between theplane, in other words that point of least resistance.

As a result using the plane to plane method in forming these structureshas some limits as to the position and number of conductive structuresformed. When a point-to-plane plane or point-to-point method is used tointroduce the electric field however, more discrete in position andnumber of conductive structures would be formed as each point has itsown electric field which is not readily dissipated when nearbyconductive structures are formed.

This was demonstrated by using a lab corona treating device on aconductive substrate that was grounded. The corona treating device actedlike a series of point sources to a plane receiving substrate. Whatresulted was a uniformly distributed conductive structure across thesurface of the adhesive.

The testing of the stability of the in situ formed electricallyconductive structures was accomplished by placing post DOR test samplesin an oven at 160° F. (71° C.) for 16 hours and retesting the impedance(AAMI EC12-2000-4.2.2.1.) and signal receptive properties. In all casesthe samples maintained the lower impedance. The conductive particles maybe in the form of carbon, and may be provided in a concentration greaterthan 1% on solids, dry weight.

The use of the composites of the present invention further provides thata conductive layer (such as conductive layers 26 and 28 of FIG. 4) thatadjoins the composite for providing a voltage to an electrode, forexample, does not need to be formed of an expensive material such assilver/silver chloride (Ag/AgCl) as is required with hydrogels.Hydrogels require such specialized conductive layers because the ionicconductivity of the hydrogel must ionically couple to the electrode. Inaccordance with the present invention on the other hand, a conductivelayer adjacent the composite may be formed of an inexpensive depositedlayer (e.g., vacuum deposited or sputter coated) of, for example,conductive particles such as those discussed above but in a higherconcentration to form a conductive layer upon deposition. Such lessexpensive materials may be used for the conductive layer because themechanism for conduction is not ionic conductivity.

As shown in FIG. 11, composites of further embodiments of the inventionmay undergo electrophoresis in multiple directions. For example, acomposite 60 may include particles having very large aspect ratios(upwards of 1000 to 1) such as carbon nanotubes 62 dispersed within adielectric material 64 as shown in FIG. 11. In the presence of anelectric field that is applied in the Z direction (as shown at 66 inFIG. 12), the particles agglomerate but because the particles are solong, they become entangled with one another when agglomeration occurs.This results in the particles not only providing electrical conductivityin the Z direction, but also providing electrical conductivity in the Xand Y directions as well due to the entangled mass of particlesextending in the X and Y directions as well as the Z direction as shownin FIG. 12.

EXAMPLE 5

In accordance with a further example therefore, an adhesive mixtureincluding FLEXcon's V-95 acrylic adhesive, a polar material (ArquadHTL-8 sold by AkzoNobel, 20% solids on solids of the V-95 adhesive, and0.04% single walled semi-conductive carbon nanotubes (CNTs). The mixturewas provided in a 3% solids paste in a 72/28 solvent blend isopropylalcohol/n-butyl alcohol (sold by Southwest Nanotechnologies of 2501Technology Place, Norman, Okla. The mixture was sonicated for 30 minutesto evenly disperse the CNTs throughout the adhesive/arquad premixture.

The mixture was then coated, dried and cured as discussed above to a 2mil (50 micron) dried thickness. The adhesive composites were made andtested as discussed above. The results were that the pre-DOR test (asper EC12-2000-4.2.2.1) showed an impedance of 100 k Ohms. The DOR test(as per EC12-2000-4.2.2.4) was pass, and that the impedance postEC12-2000-4.2.2.1 was 5 K Ohms. The signal receptivity was tested as inExample 1 to be both 95% before and after DOR. The anisotropy test asdiscussed above with respect to Example 3, found that there was an X andY conductivity component to the composite post DOR. It is expected thatmore uniform istropic conductive coatings may be formed.

Applications calling for a conductive polymeric contact material such asa sealing or attaching material to bring an EMF shield to ground, andnew ways of making membrane switch devices may all benefit fromcomposites of the present invention. Other applications that require ormay benefit from a conformable electrical contact where the interfacebetween the electrode and an active layer (such as in photovoltaics ororganic light emitting diodes) may employ composites of the presentinvention. Moreover, the possibility of using substantially lowerconcentrations of conductive particles such as nano-conductiveparticles, provides the possibility of developing clear conductivecoatings.

EXAMPLE 6

As noted above, for pressure sensitive adhesives, if the particleconcentration is high enough to form a network in whichparticle-to-particle contact is maintained then there is little chancethat the dielectric material (e.g., elastomer) of the adhesive componentis present in high enough concentrations to flow out to makesurface-to-surface contact between the substrates and an electrode,i.e., act as an adhesive. In a further example, to the dielectricmaterial of Sample 1 (the V-95 PSA and polar material) was added 25% byweight of the carbon particles of Sample 1. The composite was thencoated and dried onto a polyester based siliconized release liner to a 2mil (50 micron) dry deposition. The resulting coating had substantiallyno measurable PSA properties (tack, peel, shear). An electricallyconductive network, however, had formed in the composite, and thiscomposite was found to have a DC resistance of about 100 Ohms bothbefore and after electrophoresis.

Those skilled in the art will appreciate that numerous modifications andvariations may be made to the above disclosed embodiments withoutdeparting from the spirit and scope of the present invention.

What is claimed is:
 1. A composite having a thickness in a z directionthat is substantially less than a length in an x direction and a widthin ay direction, said composite comprising: a first dielectric materialand a second conductive material that is substantially dispersed withinthe first dielectric material, said first dielectric material being ahigh viscosity viscoelastic material having a viscosity of at least fiveorders of magnitude higher than 10 cp, said second conductive materialincluding particles having a surface energy greater than a surfaceenergy of the first dielectric material, said particles beingsubstantially smaller than the thickness of the composite, and saidparticles remaining suspended within said first dielectric material in aconcentration insufficient to provide particle-to-particle electricalconductivity through the composite in the z direction.
 2. The compositeas claimed in claim 1, wherein said first dielectric material includes apolymeric material.
 3. The composite as claimed in claim 1, wherein saidparticles are formed of any of carbon powder, flakes, granules ornanotubes.
 4. The composite as claimed in claim 3, wherein the carbon isin the form of graphite.
 5. The composite as claimed in claim 1, whereinsaid particles have densities within the range of about 0.35 g/cm³ andabout 1.20 g/cm³.
 6. The composite as claimed in claim 1, wherein saidparticles have densities within the range of about 0.5 g/cm³ and about1.0 g/cm³.
 7. The composite as claimed in claim 1, wherein saidparticles are randomly distributed within the composite prior toapplication of the electric field.
 8. A composite having a thickness ina z direction that is substantially less than a length in an x directionand a width in a y direction, said composite comprising a highlyviscoelastic dielectric material; a conductive material dispersed withinthe dielectric material, said conductive material including particlessubstantially smaller than the thickness of the electrically conductivecomposite, and said particles remaining suspended within said dielectricmaterial in a concentration insufficient to provide particle-to-particleelectrical conductivity through the composite in the z direction; andfirst and second electrically conductive electrodes on either side ofthe dielectric material such that an electric field applied between thefirst and second electrically conductive electrodes and through thecombined conductive and dielectric material, results in the formation ofat least one electrically conductive path of the particles through thecomposite along the direction of the applied electric field between thefirst and second conductive materials, said electrically conductive pathbeing formed by at least a portion of the conductive material while thedielectric material remains highly viscoelastic, having a viscosity ofat least five orders of magnitude higher than 10 cp.
 9. The composite asclaimed in claim 8, wherein said dielectric material includes an acrylicpressure sensitive adhesive.
 10. The composite as claimed in claim 8,wherein said particles are formed of any of carbon powder, flakes,granules or nanotubes.
 11. The composite as claimed in claim 10, whereinthe carbon is in the form of graphite.
 12. The composite as claimed inclaim 8, wherein said particles have densities within the range of about0.35 g/cm³ and about 1.20 g/cm³.
 13. The composite as claimed in claim8, wherein said particles have densities within the range of about 0.5g/cm³ and about 1.0 g/cm³.
 14. The composite as claimed in claim 8,wherein said particles are randomly distributed within the compositeprior to application of the electric field.
 15. An electricallyconductive composite having a thickness in a z direction that issubstantially less than a length in an x direction and a width in a ydirection, said electrically conductive composite comprising:electrically conductive particles dispersed within an acrylic pressuresensitive adhesive, said electrically conductive particles beingsubstantially smaller than the thickness of the electrically conductivecomposite, and said electrically conductive particles remainingsuspended within acrylic pressure sensitive adhesive in a concentrationinsufficient to provide particle-to-particle electrical conductivitythrough the acrylic pressure sensitive adhesive in the z direction; andat least one electrically conductive path formed through the acrylicpressure sensitive adhesive along a direction of an applied electricfield, said electrically conductive path being formed by at least someof the electrically conductive particles material while the acrylicpressure sensitive adhesive remains highly viscoelastic and suitable foruse as a pressure sensitive adhesive, and wherein the acrylic pressuresensitive adhesive has a viscosity of at least five orders of magnitudehigher than 10 cp.
 16. The electrically conductive composites as claimedin claim 15, wherein said acrylic pressure sensitive adhesive isrepresented as:

where R is an ethyl, or a butyl or a 2-ethylhexyl and n is a number ofrepeating units.
 17. The electrically conductive composite as claimed inclaim 15, wherein said electrically conductive particles are formed ofany of carbon powder, flakes, granules or nanotubes.
 18. Theelectrically conductive composite as claimed in claim 15, wherein thecarbon is in the form of graphite.
 19. The electrically conductivecomposite as claimed in claim 15, wherein said electrically conductiveparticles have densities within the range of about 0.35 g/cm³ and about1.20 g/cm³.
 20. The electrically conductive composite as claimed inclaim 15, wherein said electrically conductive particles have densitieswithin the range of about 0.5 g/cm³ and about 1.0 g/cm³.
 21. Theelectrically conductive composite as claimed in claim 15, wherein theelectrically conductive particles are randomly distributed within thecomposite prior to application of the electric field.
 22. Theelectrically conductive composite as claimed in claim 15, wherein saidelectrically conductive composite includes a plurality of independentconductive paths through the acrylic pressure sensitive adhesive alongthe direction of the applied electric field.